In order to obtain high quality of the weld and reduced costs for a welded article, today, use is made of automated welding processes in which various types of sensors measure, for example, the position of the welding joint in front of the weld, gap and misalignment between the parts which are to be welded together, or the width of the welding joint as a basis for controlling the welding process. It is previously known to make use of video cameras, for example, an analog or digital CCD camera, in order to create an image of the welding area. Use is also made of a computer with a real-time image-processing program, which is connected to the camera. This makes possible automatic measurement in the image. With the aid of the measured information, a welding robot or a welding apparatus is controlled, it being possible to regulate the welding process online.
The light from the arc and the heat radiation (blackbody radiation) from the melt must be attenuated in order for it to be possible to see or detect the melt, and details in the melt, with the camera. The image created by the camera will otherwise be unevenly exposed on account of the highly luminous area around the arc and the melt. According to previously known systems, there are problems associated with producing a precise image of the welding area with a great amount of detailed information.
A previously known such device for monitoring the welding area is described in Japanese Patent Application, JP 11187111. Therein, use is made of a stroboscope that emits ultraviolet radiation so as to illuminate the welding area. Furthermore, a camera is arranged in order to reproduce the welding area. The shutter of the camera is synchronized with the stroboscope.